Cesium beam tube detector with niobium ionizer



June 28, 1966 J. GEORGE 3,258,713

CESIUM BEAM TUBE DETECTOR WITH NIOBIUM IONIZER Filed May 28, 1965INVENTOR.

J A M ES GEO RG E ATTORNEYS United States Patent 3,258,713 CESIUM BEAMTUBE DETECTOR WITH NIOBIUM IONIZER James George, Swampscott, Mass.,assignor to National Company, Inc., Malden, Mass., a corporation ofMassachusetts Filed May 28, 1963, Ser. No. 287,735 2 Claims. (Cl. 331-3)My invention relates to an improved cesium beam tube detector and inparticular concerns a niobium ionizing detector.

In a molecular beam such as that described in U.S.

1 Patent 2,972,115 to J. .R. Zacharias et al., issued on February 14,1961, an ionizer detector is employed to convert the neutral atomic beaminto ions for conversion into an electrical signal. Ionization of thebeam of cesium atoms is accomplished by a ribbon of heated tungsten inthe beam path. Cesium atoms on striking the heated surface of thetungsten ribbon are converted and re-evaporated as positive cesium ions.A mass spectrometer is used to separate residual impurities such aspositive alkali metal ions from the positive cesium ions beforedirecting the cesium ions to the anode of a staged electron multiplier.

The conversion of cesium atoms to cesium ions in a cesium beam tubeapparatus has generally been accomplished by ionizer-detectors or hotwire detectors in which heated platinum, tungsten or tantalum wires orribbon filaments have been used. Although effective, these materialshave also required the use of mass spectrometers to remove metallicimpurities converted into ions with the cesium. For example, a heatedtungsten wire generates as an impurity potassium ions which degrade thesignal to noise ratio unless removed by mass spectrometer means.Additionally, the variation of heater energy to the electrically heatedtungsten ribbon or thermal or mechanical shocks would tend to producebursts of potassium ion emissions. Furthermore, outgassing of thedetector was often required during manufacture of the beam tubes toreduce the production of residual ion impurities by the detector.

It is an object of my invention to provide an improved ionizer detectorfor use in a cesium beam tube apparatus, which detector will eliminatethe requirement for a mass spectrometer to remove potassium ionimpurities, will significantly inhibit potassium ion bursts, andsimplify the manufacture of a beam tube apparatus by reducing the needfor outgassing the ionizer detector.

Other objects and advantages will be apparent to those skilled in theart from the following more detailed description of my invention whentaken in conjunction with the accompanying drawing which is a schematiccross sectional elevation of a niobium ribbon detector in a molecularbeam apparatus.

I have found that the objects of my invention are achieved by the use ofniobium as the ionizing surface of an ionizer-detector in a cesiummolecular beam apparatus. It has been found, quite unexpectedly, thatniobium unlike tungsten does not produce bursts of potassium ions onvariation of the heater power or by mechanical shocks such as excessvibration. The employment of niobium has also significantly improved thesignal to noise ratio.

A niobium ionizing surface is shown in the drawing which illustrates anionizer detector 12 at the one end of a conventional cesium beam tube 36where the cesium beam emerges from the B magnet field. The tube 36 isattached to and communicates with the evacuated detector chamber 116.The detector is of a completely sealed construction. Attached to thewall 114 are electrostatic plates 117 and 118. A narrow slit 120 in theplates per- "ice mits the cesium atoms entering the detector chamber tostrike the ionizing ribbon 122. The ionizing ribbon is a heated niobiumribbon filament which has a long axis aligned with the long axis of thecross-section of the beam of atoms. The width of the ionizing ribbon issuch that it will not be struck except by those atoms in that portion ofthe beam area in which the atoms of the (4, 0) energy level are located.This hot ribbon is a surface ionizer; that is, neutral Cs particlesstrike the surface, are adsorbed, and quickly reevaporate as singlycharged positive ions. As shown the ribbon 122 is mounted at a 30 anglefrom the normal of the cesium atoms striking the surface. Afterionization, the particles are accelerated to an energy of about 15 e.v.thru the parallel plate system 117, 118. The particles are deflectedthrough a 30 angle and subsequently accelerated by a parallel plate lens126, to enter a 14 stage electron multiplier 128 which develops anelectrical output signal. An ion pump 130 communicates with the detectorchamber 116 to remove cesium atoms and gaseous impurities and tomaintain the vacuum desired.

I have directly compared the performance of a tungsten and niobiumionizing surface in a conventional cesium beam tube apparatus. Thecesium beam tube comprises a cesium oven source at the one end of anevacuated beam tube with the ionizer-detector at the other end. The beamtube included: a collimator to form the heated cesium atoms into a beamof predetermined form; externally mounted C-shaped A and B magnets toproduce a strong inhomogeneous A and B magnetic field; and means tocreate an intermediate weak homogeneous C magnetic field; with a dualcavity microwave guide structure in the weak C field whereby the cesiumatoms passing through the cavities are subject to an intense oscillatingmagnetic field.

The ionizer detector apparatus included a supported ionizing surface ofniobium formed in a ribbon channel to provide mechanical strength andmade of 99.9% pure niobium metal of 0.0011 thickness. The detector isaligned with the beam of cesium atoms from the B magnetic field and isin an evacuated region of 10"" mm. of mercury or less. The long axis ofthe niobium ribbon is aligned with the long axis of the beamcross-section. In front of and to the rear of the niobium ribbon weredisposed parallel electrostatic plates with a narrow slit in the frontplate to permit a predetermined portion of the neutral cesium beam tostrike the niobium ionizing surface. With the niobium ionizing surface amass spec trometer was not required, rather the positive cesium ionsfrom the ionizing surface were accelerated through the parallel platesto the anode of a 10 or more stage electron multiplier to convert thecesium into a proportional elec tric D.C. signal.

This signal is handled by a servo control system such as described inU.S. Patent 2,883,546 to E. F. Grant, issued April 21, 1959; U.S. Patent2,960,663 to W. A. Mainberger, issued November 15, 1960, and U.S. Patent2,994,836 to J. H. Holloway, issued August 1, 1961. The control systemto maintain the frequency as close as pos sible to the desired frequencycommonly includes: a basic frequency oscillator, one output going to amultiplier to give a frequency of 9180 me. and the other output to asynthesizer to provide a frequency of 12.631840 maps. The output of thesynthesizer is frequency modulated at about c.p.s. by a generator. Powerfor the RP. excitation of the beam tube is obtained by a glystronoscillator, the output of which is mixed and amplified to obtain thefrequency of the atomic resonance by a means now well known.

The niobium ionizing surface was heated to about 800 to 1200 C. inoperation by the use of the niobium ribbon channel as a resistanceelement in electrical heating circuit with the niobium being inelectrical communication with a power supply. The temperature of theniobium ionizing surface is commonly kept sufiiciently high to followthe modulations of the RF. signal used in the beam tube. Thus, if thecesium atoms are perturbed out of the microwave structure at 100 cyclesper second, the temperature of the ionizing surface should besufficiently high so that the cesium atoms spend less than of a secondon the surface. The heated niobium surface thus controls the rate ofemission of the cesium ions. Further elevated temperatures keep thereduction of any oxides of niobium at a high rate which aids inpromoting a relatively constant work function for the niobium surface.

The niobium ionizing surface can be any form or shape such as a hollowrod with external heating of the rod accomplished by separate heatingmeans, or can be in the form of solid cylinders, etc.

In operation the neutral cesium atoms on striking the heated niobiumsurface are adsorbed and quickly converted to positive cesium ions andthen boiled off as ions, which pass through the plates into the electronmultiplier. In an evacuated cesium beam tube apparatus employing aniobium heated ionizing surface, I have found that despite mechanicalshocks and variation in the electric heater power supply, bursts ofpotassium ion emissions were not observed. Further extensive outgassingof the detector was not required for satisfactory operation. In the beamtube apparatus satisfactory operation was accomplished without the needfor a bulky and costly mass spectrometer to separate potassiumimpurities. In normal operation with a 99.9% pure niobium element,potassium ion emission was found to represent less than 10- amps or 10ion particles/ sec.

Operation of a cesium beam tube with a conventional heated tungstenionizing surface required, the use of a mass spectrometer, and gavepotassium ion bursts on thermal or mechanical shocks to the detector.

My invention thus considerably improves the operation of a cesiummolecular beam tube apparatus by reducing undesired potassium emissionby a factor of or more, while avoiding potassium emission bursts. Asdescribed, my invention provides for a significant improvement in useand the cost of manufacturing cesium beam tube apparatus.

Having thus described my invention, I claim:

1. An atom ionizer which includes a heated element consistingessentially of niobium in the path of a beam of atoms.

2. A detector which includes: an evacuated detector chamber adapted toreceive a beam of atoms; means to maintain the vacuum in the chamber; aheated ionizing surface consisting essentially of pure niobium in thepath of the beam; and means to convert the ionized atoms into anelectrical output signal.

References Cited by the Examiner UNITED STATES PATENTS 10/1960 Reder331-3 2/1962 Zacharias et al 3313 OTHER REFERENCES ROY LAKE, PrimaryExaminer.

S. H. GRIMM, Assistant Examiner.

1. AN ATOM IONIZER WHICH INCLUDES A HEATED ELEMENT CONSISTINGESSENTIALLY OF NIOBIUM IN THE PATH OF A BEAM OF OTOMS.